Batteries have been utilized for many years as electrical power generators in remote locations. Through the controlled movement of ions between electrodes (anode and cathode), a power circuit is generated, thereby providing a source of electricity that can be utilized until the excess ions in one electrode are depleted and no further electrical generation is possible. In more recent years, rechargeable batteries have been created to allow for longer lifetimes for such remote power sources, albeit through the need for connecting such batteries to other electrical sources for a certain period of time. All in all, however, the capability of reusing such a battery has led to greater potentials for use, particularly through cell phone and laptop computer usage and, even more so, to the possibility of automobiles that solely require electricity to function.
Such batteries typically include at least five distinct components. A case (or container) houses everything in a secure and reliable manner to prevent leakage to the outside as well as environmental exposure inside. Within the case are an anode and a cathode, separated effectively by a separator, as well as an electrolyte solution (low viscosity liquid) that transport ions through the separator between the anode and cathode. The rechargeable batteries of today and, presumably tomorrow, will run the gamut of rather small and portable devices, but with a great deal of electrical generation potential in order to remain effective for long periods between charging episodes, to very large types present within automobiles, as an example, that include large electrodes (at least in surface area) that must not contact one another and large number of ions that must consistently and constantly pass through a membrane to complete the necessary circuit, all at a level of power generation conducive to providing sufficient electricity to run an automobile motor. As such, the capability and versatility of battery separators in the future must meet certain requirements that have yet to be provided within the current industry.
Generally speaking, battery separators have been utilized since the advent of closed-cell batteries to provide necessary protection from unwanted contact between electrodes as well as to permit effective transport of electrolytes within power generating cells. Typically, such materials have been of film structure, sufficiently thin to reduce the weight and volume of a battery device while imparting the necessary properties noted above at the same time. Such separators must exhibit other characteristics, as well, to allow for proper battery function. These include chemical stability, suitable porosity of ionic species, effective pore size for electrolyte transfer, proper permeability, effective mechanical strength, and the capability of retaining dimensional and functional stability when exposed to high temperatures (as well as the potential for shutdown if the temperature rises to an abnormally high level).
In greater detail, then, the separator material must be of sufficient strength and constitution to withstand a number of different scenarios. Initially, the separator must not suffer tears or punctures during the stresses of battery assembly. In this manner, the overall mechanical strength of the separator is extremely important, particularly as high tensile strength material in both the machine and cross (i.e., transverse) directions allows the manufacturer to handle such a separator more easily and without stringent guidelines lest the separator suffer structural failure or loss during such a critical procedure. Additionally, from a chemical perspective, the separator must withstand the oxidative and reductive environment within the battery itself, particularly when fully charged. Any failure during use, specifically in terms of structural integrity permitting abnormally high amounts of current to pass or for the electrodes to touch, would destroy the power generation capability and render the battery totally ineffective. Thus, even above the ability to weather chemical exposure, such a separator must also not lose dimensional stability (i.e., warp or melt) or mechanical strength during storage, manufacture, and use, either, for the same reasons noted above.
Simultaneously, however, the separator must be of proper thickness to, in essence, facilitate the high energy and power densities of the battery, itself. A uniform thickness is quite important, too, in order to allow for a long life cycle as any uneven wear on the separator will be the weak link in terms of proper electrolyte passage, as well as electrode contact prevention.
Additionally, such a separator must exhibit proper porosity and pore sizes to accord, again, the proper transport of ions through such a membrane (as well as proper capacity to retain a certain amount of liquid electrolyte to facilitate such ion transfer during use). The pores themselves should be sufficiently small to prevent electrode components from entering and/or passing through the membrane, while also allowing, again, as noted above, for the proper rate of transfer of electrolyte ions. As well, uniformity in pore sizes, as well as pore size distribution, provides a more uniform result in power generation over time as well as more reliable long-term stability for the overall battery as, as discussed previously, uniform wear on the battery separator, at least as best controlled in such a system, allows for longer life-cycles. It additionally can be advantageous to ensure the pores therein may properly close upon exposure to abnormally high temperatures to prevent excessive and undesirable ion transfer upon such a battery failure (i.e., to prevent fires and other like hazards).
As well, the pore sizes and distributions may increase or decrease the air resistance of the separator, thus allowing for simple measurements of the separator that indicate the ability of the separator to allow adequate passage of the electrolyte present within the battery itself. For instance, mean flow pore size can be measured according to ASTM E-1294, and this measurement can be used to help determine the barrier properties of the separator. Thus, with low pore size, the rigidity of the pores themselves (i.e., the ability of the pores to remain a certain size during use over time and upon exposure to a set pressure) allows for effective control of electrode separation as well. More importantly, perhaps, is the capability of such pore size levels to limit dendrite formation in order to reduce the chances of crystal formation on an anode (such a lithium crystals on a graphite anode) that would deleteriously impact the power generation capability of the battery over time.
There is also great concern with the dimensional stability of such a separator when utilized within a typical lithium ion cell, as alluded to above. The separator necessarily provides a porous barrier for ion diffusion over the life of the battery, certainly. However, in certain situations, elevated temperatures, either from external sources or within the cell itself, may expose susceptible separator materials to undesirable shrinking, warping, or melting, any of which may deleteriously affect the capability of the battery over time. As such, since reduction of temperature levels and/or removal of such battery types from elevated temperatures during actual utilization are very difficult to achieve, the separator itself should include materials that can withstand such high temperatures without exhibiting any appreciable effects upon exposure. Alternatively, the utilization of combinations of materials wherein one type of fiber, for instance, may provide such a beneficial result while still permitting the separator to perform at its optimum level, would be highly attractive.
Furthermore, the separator must not impair the ability of the electrolyte to completely fill the entire cell during manufacture, storage and use. Thus, the separator must exhibit proper wicking and/or wettability during such phases in order to ensure the electrolyte in fact may properly transfer ions through the membrane; if the separator were not conducive to such a situation, then the electrolyte would not properly reside on and in the separator pores and the necessary ion transmission would not readily occur. Additionally, it is understood that such proper wettability of the separator is generally required in order to ensure liquid electrolyte dispersion on the separator surface and within the cell itself. Non-uniformity of electrolyte dispersion may result in dendritic formations within the cell and on the separator surface, thereby creating an elevated potential for battery failures and short circuiting therein.
Additionally, the efficiency of an electrolyte to continuously and reliably generate sufficient electrical power within a battery cell is highly dependent on the delivery rate thereof across a separator membrane. Typically, to allow for better efficiencies in such a situation, electrolyte formulations include multiple solvents, rather than a single type. In that manner, the multiple solvent solution allows for the depletion of one electrolyte with compensation by another. The different chemical structures, however, tend to alter the level of electrical generation in such a scenario, at least at times, and the ability to provide a stable membrane transfer (and then return to create a charged battery) may be compromised through this necessary combination of species. In any event, the potential to allow for greater amounts of certain electrolyte structures without reducing the overall effectiveness and thus the potential to accord a longer period of charge depletion before charging is necessary, would be highly prized within this industry. To date, the only way such has been possible is the utilization of a pre-gelled electrolyte solution that is rather difficult to properly introduce within a battery cell and, consequently, requires a certain time period to properly constitute over and through a battery separator. Likewise, with typical polypropylene structures, gelled electrolytes must be provided in this external fashion and do not readily react for long-term stability. All in all, the basic problems faced with the known beneficial properties accorded the industry with gelled electrolyte formulations have not been improved upon in the past. Although the ability to potentially, at least, control electrolyte membrane transfer through a gelled state is significant, the problems associated with such a gelling requirement have yet to optimize such a platform within the battery industry.
To date, however, as noted above, the standards in place today do not comport to such critical considerations, particularly as it concerns any manner of improving the capability of gelling an electrolyte formulation in situ, rather than externally for cell introduction thereafter. The general aim of an effective battery separator is to provide such beneficial characteristics all within a single thin sheet of material; no discussion has ever been made as to the electrolyte gelling capacity of such an already in-place battery component. As such, there exists a significant need to improve upon the methods of incorporating gelled electrolyte formulations within battery cells for greater efficiency, longer charge capacity, and easier application during manufacture, at least. Combined with the potential for a suitable battery separator that exhibits proper pore size, dimensional stability, and other versatile end results (i.e., air resistance, for example) as well as necessary levels of mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, and meltdown properties, the potential for a one-size-fits-all approach would be of great interest within the rechargeable battery separator industry. As of today, nothing has been provided to that extent.